Since the advent of a resist for KrF excimer laser (248 nm), an image forming method called chemical amplification is used as an image forming method for resists so as to compensate for sensitivity reduction due to light absorption. For example, the image forming method by positive chemical amplification is an image forming method of causing an acid generator to decompose in the exposed area upon exposure to an excimer laser, an electron beam, extreme-ultraviolet light or the like and produce an acid, converting an alkali-insoluble group into an alkali-soluble group by using the generated acid as a reaction catalyst in the baking after exposure (PES: Post-Exposure Baking), and removing the exposed area with an alkali developer.
As for the alkali developer used in the method above, various alkali developers have been proposed, but an aqueous alkali developer of 2.38 mass % TMAH (an aqueous tetramethylammonium hydroxide solution) is being used for general purposes.
On the other hand, as well as the currently predominant positive resist, a negative chemical amplification resist composition for use in the pattern formation by alkali development is also being developed (see, for example, JP-A-2006-317803 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-2006-259582, JP-A-2006-195050 and JP-A-2000-206694). Because, in the manufacture of a semiconductor device or the like, patterns having various profiles such as line, trench and hole need to be formed, but some patterns are difficult to form by the current positive resist.
Also, a double developing technique as a double patterning technology for further raising the resolution is described in JP-A-2008-292975. When a resist film is exposed, the polarity of a resin in the resist composition becomes high in the high light intensity region and is kept low in the low light intensity region, and by making use of this property, the high exposure region of a specific resist film is dissolved with a high-polarity developer, whereas the low exposure region is dissolved with an organic solvent-containing developer, as a result, the medium exposure dose region remains without being dissolved/removed by the development and a line-and-space pattern having a pitch half the pitch of the exposure mask is formed.
Such a resist composition is often used by applying it on a bottom anti-reflection coating formed on a substrate. The bottom anti-reflection coating is used to prevent diffused reflection of an actinic ray on the substrate or reduce the effect of a standing wave generated due to incident light and reflected light, but in the microfabrication such as ion implantation, a bottom anti-reflection coating cannot be formed in some cases.
The technology of using a resist composition to block ion when implanting ion (charge injection), which is one step of the logic device fabrication or the like, plays an important role. For example, JP-A-2008-197606 discloses a technique where a resist film is formed using a radiation-sensitive composition for ion implantation, the composition containing a resin having two kinds of specific repeating units and containing an acid-dissociative group, and the resist film is exposed by an ArF excimer laser exposure apparatus and then developed with an aqueous alkali developer of 2.38 mass % TMAH, whereby excellent ion blocking property and excellent resistance to fracture of a resist are obtained, and even when a bottom anti-reflection coating is not formed on a substrate, a resist film having good sensitivity and resolution and ensuring a good pattern profile and a small pattern fluctuation range is formed.
In the case of using a resist composition for ion implantation, the resist composition is sometimes applied, exposed and developed on a substrate having previously formed thereon a pattern (hereinafter, referred to as a stepped substrate), and microfabrication on a stepped substrate is required. In the patterning used for ion implantation, one of tasks difficult to solve is the problem of scum on the substrate, which is generated after development due to bad removability of the film formed from the resist composition. In the case of developing the resist film with an aqueous alkali developer, the exposed area of the resist film is dissolved with the developer and therefore, for allowing no remaining of a resist film residue, sufficient light must be applied to the bottom of the resist film, but when such light is applied, the effect of reflection by the substrate becomes strong, and the resist film in the unexposed area, which should remain as a pattern, is exposed to the reflected light, causing a problem that the bottom of the pattern falls off. Accordingly, when the resist film is developed with an aqueous alkali developer, it is difficult to solve the problem of scum on the substrate while keeping a good pattern profile. Particularly, in the patterning on the stepped substrate above or the patterning without forming a bottom anti-reflection coating on a substrate, the effect of light reflection by the substrate becomes strong, and the problem of scum generated on the substrate after development is serious.